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The allowable drift Da is calculated according to ASCE 7 Table 12.12-1 for seismic design and is input in the Design Settings for wind design. The default wind value is to 1/500 story height, which is midway between the 1/400 and 1/600 limits suggested in ASCE 7 CC.2.2.
For seismic design, it is assumed that the wall and ceiling systems have been designed to accommodate story drifts, so that the first line in the Table 12.2-1 is used for structures four stories or less, and the line for "All other structures" used for 5-and 6-level structures.
The drift limit in feet or meters is shown in the Story Drift table for each level of the building.
Story height
The story height hsx for each level is the wall height plus the upper floor thickness. The wall height on the gable end of the structure extends to the eave only, so that the triangular portion of the gable end is neglected. (The deflection calculation used to determine story drift assumes rectangular walls, and it can be assumed that the roof as a unit deflects the same amount as its juncture with the supporting wall).
Risk Category
The Risk category is input in the Site Information dialog.